1. Field of the Invention
The present invention relates to electrical amplifiers producing a high voltage output in a short rise time. More specifically, the present invention relates to high voltage electrical amplifiers that are computer controlled and have a short rise time.
2. Description of Related Art
As performance requirements of electrical and optical systems increase, the need has arisen for ever faster electronic control systems. The need for higher speeds of operation, and the type of equipment that can process electrical signals at that speed is typically related to the frequency of the signal that is to be processed; electrical equipment that amplifies audio signals (in the range of 20 Hz to 20 KHz) is far different from electrical equipment that processes microwave signals (1 GHz to 1000 GHz). For some applications, an electrical pulse is desirable. The sharpness of this pulse is determined in part by the frequency response of the electrical circuit that produces the pulse. A sharper pulse can be produced by a circuit that has a wider frequency response that includes higher frequencies; this circuit can also be characterized as having a faster rise time.
The need for electronic control of laser optical systems has created a great need for circuits having a fast rise time. The word "LASER" is actually an acronym for "Light Amplification by Stimulated Emission of Radiation". Laser light is electromagnetic radiation having a frequency much higher than microwave radiation, around 10.sup.15 Hz, equalling 1,000,000 GHz. Typically laser light is not defined in terms of frequency, but rather is described in terms of its wavelength in free space. For example, a standard HeNe laser has an output wavelength of 0.6328 .mu.m in free space.
A common component in an optical system is an electro-optic modulator. This component is a crystal comprised of a material such as potassium dideuterium phosphate, known as KD*P. To modulate the laser light in the system, a large voltage is applied across the crystal. The applied voltage affects the indices of refraction along the axes of the electro-optic crystal. KD*P requires a voltage in the range of several kilovolts (KV) for effective modulation. It is often desirable to control the voltage as precisely and quickly as possible, in order to provide the necessary electro-optic response. A common application of electro-optic components is in Q-switching, where the electro-optic modulator is used to introduce cavity losses in an amount large enough to prevent laser oscillation and therefore a laser gain medium can be pumped to store a large amount of energy. Then, the Q-switch is actuated to allow laser oscillation in the laser cavity, and a high power pulse is suddenly released using the energy stored in the gain medium.
A typical electro-optic Q-switch is actuated by a voltage pulse of several KV that is applied as quickly as possible. Speed of operation is in part determined by how quickly the electronic circuit can apply the high voltage, which is determined by the rise time of the circuit, and affected by the capacitance of the system. Thus there is a need for an electronic circuit that has a fast rise time, while providing an output with a voltage in the KV range with a power necessary to overcome the capacitance of the load.
To obtain quick and precise control over the electro-optic modulator, it is desirable to connect it to a control system that can be programmed to provide any desired output waveform. For example, a pulse shaped in a particular manner may provide the optimum Q-switch in one laser application, and a pulse shaped in another way may provide the optimum Q-switch for another laser application. A computer, easily programmed, can provide a great deal of flexibility in determining the shape of the pulse. This shape can be determined in a program, and if a different pulse is desired, it is necessary only to change the program that determines the pulse.
Digital computers are available that have a response time as fast as several nanoseconds. These computers can function as a control circuit that determines the shape of a pulse. For example discrete digital integrated circuits based on the ECL (Emitter-Coupled Logic) technology have a response time of one or two nanoseconds. ECL components can be combined together by a digital designer to obtain a circuit having a response of several nanoseconds. This circuit can be connected to a standard high speed D/A (digital to analog) converter so that a digital number is converted to a discrete voltage level. However, integrated circuits (ICs) have an output that is limited in voltage and power. Typically the output of an IC will be no more that a few volts, far below the voltage necessary to operate an electro-optic modulator. For example, the digital designer may choose an output of 1.0 volt for ECL as the maximum voltage. Therefore, the output will vary between 0.0 volts and 1.0 volt. As a further limitation of IC components, the power that can be safely dissipated is typically in the tens of milliwatts (mW). This very low power dissipation makes it difficult for digital components to quickly overcome any significant capacitance.
An amplifier is needed to amplify the signal from the control circuit and provide a high voltage electrical output to the electro-optic modulator with as little delay as possible. It is desirable that the amplifier be stable at high frequencies, have a substantial gain in both current and voltage, have a fast rise time and a wide bandwidth (frequency response) including high frequencies. In some instances, feedback may be used to increase the bandwidth; however, feedback causes stability problems which affect the amplifier's ability to produce an accurate pulse. An amplifier with a fast rise time typically has stability problems that can cause an unpredictable amplifier response to high frequencies inherent in a quickly rising pulse.
A problem affecting the rise time of a circuit is its capacitance. The circuit capacitance affects the amount and time duration of current which must be applied to deposit a charge on the circuit components and thereby become operational. It is known that field effect transistors such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) can provide a fast frequency response, once their capacitance is overcome. To account for the circuit's capacitance, the input to a MOSFET may be biased in an operating range wherein the relationship between input and output is approximately linear. The actual biasing circuitry embodied in any particular circuit is determined by a variety of design criteria, such as bandwidth, response time, and stability. For electro-optic applications, and for other pulsed power applications, there is a need for a circuit that can provide a very quick rise time and be stable at the high frequencies inherent in a quickly rising pulse.
Vacuum tubes are often used to amplify and switch high power pulses. However, they are bulky and expensive. It would be an advantage to provide a high power amplification and switching circuit comprising solid state components instead of vacuum tubes.
Another problem with pulsed power amplifiers is matching the output impedance of the amplifier to the impedance of the load. If impedance is not matched, the result may be an unwanted output waveform and a loss of output power. Typical transmission lines have an impedance of 50.OMEGA., while other applications may have a different impedance.